A fuel cell is an energy conversion device that converts chemical energy into electrical energy. The fuel cell generates electricity and heat by electrochemically combining a gaseous fuel, such as hydrogen, carbon monoxide, or a hydrocarbon, and an oxidant, such as air or oxygen, across an ion-conducting electrolyte. The fuel cell generally consists of two electrodes positioned on opposite sides of an electrolyte. The oxidant passes over the oxygen electrode (cathode) while the fuel passes over the fuel electrode (anode), generating electricity, water, and heat.
A solid oxide fuel cell (SOFC) is constructed entirely of solid-state materials, utilizing an ion conductive oxide ceramic as the electrolyte. The electrochemical cell in a SOFC comprises an anode and a cathode with an electrolyte disposed therebetween.
In a SOFC, a fuel flows to the anode where it is oxidized by oxygen ions from the electrolyte, producing electrons that are released to the external circuit, and mostly water and carbon dioxide are removed in the fuel flow stream. At the cathode, the oxidant accepts electrons from the external circuit to form oxygen ions. The oxygen ions migrate across the electrolyte to the anode. The flow of electrons through the external circuit provides for consumable or storable electricity. However, each individual electrochemical cell generates a relatively small voltage. Higher voltages are attained by electrically connecting a plurality of electrochemical cells in series to form a stack.
In operation, a SOFC system generates electricity and heat by this electrochemical process of combining a fuel and an oxidant. The fuel (i.e., reformate) provided to the SOFC is produced in a reformer. Byproducts from the SOFC, a supply of oxidant, and a supply of reformate can be directed through a waste energy recovery unit. The waste energy recovery unit is a device that converts chemical energy and thermal energy into input thermal energy for the SOFC system. This is accomplished with heat exchangers. Unlike a SOFC, the waste energy recovery unit is comprised of durable and heat transferable materials. These waste energy recovery units have many tubes and connections for directing the chemical and thermal energy through the large unit.
The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a fuel cell system including a fuel cell stack, a reformer system, and a waste energy recovery (or heat exchanger) assembly is presented. The waste energy recovery assembly receives an anode supply and a cathode supply that are heated by exhaust gases from the fuel cell stack. The heated anode supply and cathode supply are then directed to the fuel cell stack. The waste energy recovery assembly includes a series of stacked plates. The plates have openings or manifold passages for flow of the anode supply, the cathode supply, and the cell stack exhaust. The plates also have etchings that define flow channels for the anode supply, the cathode supply, and the cell stack exhaust across the plates. The flow direction of the plates alternates from one plate to the next. These plates are alternately stacked until the desired flow area and heat transfer are achieved. The total number of plates forming a waste energy recovery assembly can range from two to several hundred, depending on space and weight restrictions, and the like. Since the direction of flow of each plate is perpendicular to the direction of flow of the next plate in series, the cool (anode and cathode) gases flow along side a plate experiencing a flow of heated exhaust gases and are thusly heated.
The above described and other features are exemplified by the following figures and detailed description.